TAMR is expected to be one of the future generations of magnetic recording technologies that will enable recording at ˜1-10 Tb/in2 data densities. TAMR involves raising the temperature of a small region of the magnetic medium to near its Curie temperature where both of its coercivity and anisotropy are significantly reduced and magnetic writing becomes easier to achieve even with weak write fields characteristic of small write heads in high recording density schemes. In TAMR, optical power from a laser diode is converted into localized heating in a recording medium during a write process to temporarily reduce the field needed to switch the magnetizations of the medium grains. Thus, with a sharp temperature gradient of TAMR acting alone or in alignment with a high magnetic field gradient, data storage density can be further improved with respect to current state of the art recording technology.
In addition to the components of conventional write heads, a TAMR head includes an optical waveguide (WG), and a plasmon generator (PG) that is also referred to as a near-field transducer. The waveguide serves as an intermediate path to guide light (from a laser diode mounted on the back of a slider) to the PG where the light optical mode couples to the propagating plasmon mode of the PG. After the optical energy is transformed to plasmon energy with energy transmission along the PG, it is concentrated at the medium location where heating is desired. Ideally, the heating spot is correctly aligned with the magnetic field from the write head to realize optimum TAMR performance.
Due to an inherent mode profile mismatch between the laser diode's far-field and the waveguide mode required to excite the near-field transducer, the waveguide's cross-sectional dimensions are commonly varied along the length of the slider so as to improve the coupling efficiency. The portion of the optical waveguide (WG) where the cross-sectional dimension changes along the light's propagation direction is typically called the spot-size converter. The spot-size converter usually includes multiple WG layers stacked on top of each other so that the total stack thickness is on the order of the laser diode spot size (around 1 micron). To achieve lateral confinement of light, the WG layers are tapered in the cross-track direction. For vertical confinement of light, all of the WG layers except the one (primary waveguide) that eventually terminates at the ABS, may be tapered in the cross-track direction to a tip with a very small cross-track dimension less than 100 nm to force the propagating light mode into the primary WG that extends all the way to the ABS. Unfortunately, current state of the art nanofabrication techniques cannot readily provide reproducible waveguide taper tips having widths of 50-100 nm or less at the required aspect ratios for typical TAMR spot-size converters. State of the art TAMR spot size converter schemes employ waveguide tapers that terminate in sharp tips. As a result, there is a large variability in spot-size converter size and efficiency from part to part.
Furthermore, TAMR sliders with low efficiency spot-size converters require a higher power output from the laser diode that causes excess heating from the laser diode, and lower reliability. Also, performance suffers from stray light that enters the slider but is not coupled through the spot size converter into the PG. Therefore, an improved WG spot-size converter is needed that is more amenable to current nanofabrication techniques so that the taper tip size is more reproducibly made to give higher yields of high efficiency TAMR devices.